Got a pic. I have a Range Rover on board diagnostic display sticking out from under the P-side seat of my DI. I don't know if it is the same or if it should even be there. It always says 34 .FWIW. I will trade it straight up for a diesel Defender 110 4 door in white.

Use this procedure to clear faults:
1. Switch "on" the ignition.
2. Disconnect the serial link mating plug, wait five seconds, and reconnect.
3. Switch "off" the ignition, and wait several seconds.
4. Switch "on" the ignition. The display should now reset.
Note: It should either show a lower priority fault code or appear dark.

When I first connected it up the screen was blank and remained so until I disconnected the MAF to induce a fault. I then started the car and of course up came code 12. However then I couldn't clear the fault. Somehow I managed to clear it later but then I fiddled some more and disconnected the MAF again so I could figure out what I did. Since then (yesterday) code 12 has remained on the screen.

I am thinking that I should be able to clear the code in order to read the next one and if there is actually a MAF fault then it should only come back once the car is started.

There is no fault with the MAF. It drives fine and there is a big difference when the MAF is physically connected and disconnected.